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Hi there thanks for offering this forum. We are doing a net zero house in the San Antonio area. We are doing a hybrid insulation of 1” spray foam (closed cell) with a loose cellulose to fill the depth of the roof truss. The insulation will be on the ceiling under a 5/8” plywood that is covered with butyl based self adhesive membrane used for hot climates. With that said my question is should we put a 1x3 12” on center (not sure what width of spacing) running ridge to roof edge with gaps between to allow vertical venting as well. I’ve talked to a metal roofing representative and they suggested a vent between the roofing membrane and the metal roof. I also talked to a builder that has been doing green homes in this area for a while and he is saying the vent is not necessary. Can you refer me to info on how to do this project based on your recommendation?
Thanks Jeff. It may be a good idea to include a vapor barrier behind your ceilings. That said, I do not think an air gap on top of the roof deck is mandatory but it would be helpful in terms of creating a thermal break that enhances energy efficiency since you are trying to maximize efficiency. That said, lots of energy efficient metal roofs are installed without that gap. If you do the gap, you will need to cross batten ... the vertical battens first and then horizontal battens following by the roofing panels. The metal roofing should not be applied to vertical battens.
Based on your climate (i.e. Texas), some above deck venting (convective cooling) in addition to a layer of foam with some radiant barrier on it would actually make the entire roof system work much more effectively. At that point, you would be warming the top chord on the trusses by moving it inside the warm wall and would decrease the likelihood of condensation significantly.
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